Haze-free BST films

ABSTRACT

Described herein is a method for producing a haze-free (Ba, Sr)TiO 3  (BST) film, and devices incorporating the same. In one embodiment, the BST film is made haze-free by depositing the film with a substantially uniform desired crystal orientation, for example, (100), preferably by forming the film by metal-organic chemical vapor deposition at a temperature greater than about 580° C. at a rate of less than about 80 Å/min, to result in a film having about 50 to 53.5 atomic percent titanium. In another embodiment, where the BST film serves as a capacitor for a DRAM memory cell, a desired {100} orientation is induced by depositing the bottom electrode over a nucleation layer of NiO, which gives the bottom electrode a preferential {100} orientation. BST is then grown over the {100} oriented bottom electrode also with a {100} orientation. A nucleation layer of materials such as Ti, Nb and Mn can also be provided over the bottom electrode and beneath the BST film to induce smooth, haze-free BST growth. Haze-free BST film can also be favored by forming the bottom electrode at high temperatures close to those used for BST deposition, and without a vacuum break between the bottom electrode and BST deposition.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to (Ba,Sr)TiO₃ (BST) thin films,and more particularly to a method for creating a haze-free BST thin filmwith a high dielectric constant.

[0003] 2. Description of the Related Art

[0004] (Ba,Sr)TiO₃ (BST) films are commonly used as dielectric materialsfor capacitors, gate dielectrics and high frequency electronic circuits.More particularly, BST films have found application as capacitors indynamic random access memory (DRAM) cells. A typical DRAM cell comprisesa charge storage capacitor (or cell capacitor) coupled to an accessdevice such as a metal-oxide semiconductor field effect transistor(MOSFET). The MOSFET functions to apply or remove charge on thecapacitor, thus affecting a logical state defined by the stored charge.The amount of charge stored on the capacitor is determined by thecapacitance, C=εε₀A/d, where ε is the dielectric constant of thecapacitor dielectric, ε₀ is the vacuum permittivity, A is the electrode(or storage node) area, and d is the interelectrode spacing. Theconditions of DRAM operation such as operating voltage, leakage rate andrefresh rate, will in general mandate that a certain minimum charge bestored by the capacitor.

[0005] BST is desirable for such applications because of its highdielectric constant, low DC leakage, low dispersion up to highfrequencies and stable operation at high temperatures. The highdielectric constant of BST thereby gives the material the ability toyield high capacitance when placed between a pair of electrodes. BSTfilms grown for applications such as DRAM capacitors are typically madeusing metal-organic chemical vapor deposition (MOCVD) or sputtering.However, MOCVD growth of such films typically leads to problems such ashaze which reduces the dielectric constant of the material and increasesleakage currents. Specifically, haze is caused by the growth ofspatially correlated, non-textured BST, which in turn createsdiscernible optical scatter and a cloudy or hazy appearance in the film.For example, haze may be created when a film desired to be grown in a(100) orientation has orientations other than (100), such as (110) or(111), thereby disrupting the texture of the film. When BST is used in acapacitor structure, haze causes its capacitance to decrease as much as50% and leakage currents to increase by a factor of 10 to 1000 withrespect to smooth films.

[0006] Furthermore, electrodes and other materials on which BST filmsare deposited often suffer from process-induced defects such as hillockformation which may severely limit performance. Hillocks are smallnodules which form when the electrode or other material is deposited orsubjected to post-deposition processing. For example, hillocks canresult from excessive compressive stress induced by the difference inthermal expansion coefficient between the BST film and the underlyingelectrode material during post-deposition heating steps. Such thermalprocessing is typical in the course of semiconductor fabrication.Hillock formation may create troughs, breaks, voids and spikes along theelectrode surface, thereby leading to uneven BST growth and stress inthe BST film.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of this invention to produce ahaze-free BST thin film with a high dielectric constant and low leakagecurrents. It is also an object of this invention to produce a BST thinfilm that has low stress. It is further an object of this invention toproduce an electrode or other base material onto which a BST film isdeposited that is smooth and hillock-free to improve the properties ofthe subsequently deposited BST film.

[0008] These objects are achieved generally through the control of oneor more processing conditions in the fabrication of the BST film.Briefly stated, haze can be reduced by increasing the BST depositiontemperature, decreasing the deposition rate and increasing the atomicpercent of titanium in the BST film. These conditions favor theformation of a highly textured film, i.e., a film with a substantiallyuniform desired crystal orientation. Furthermore, use of highly texturedsubstrates, bottom electrodes or nucleation layers also favors haze-freeand low stress BST films. Moreover, the above-stated objects areachieved by forming a substrate layer such as a bottom electrode and theBST film in a clustered tool.

[0009] In one aspect of the present invention, a method of forming ahaze-free BST film over a substrate assembly is provided. The methodcomprises supplying BST sources into a chamber, and inducing texturedgrowth of the BST film over the substrate assembly in a substantiallyuniform desired crystal orientation. In one preferred embodiment, theBST film is deposited at a rate of less than about 80 Å/min at a chambertemperature above about 580° C. The BST film is preferably grown usingmetal-organic chemical vapor deposition (MOCVD), and results in a filmhaving a concentration of about 50 to 53.5 atomic percent titanium.

[0010] In another aspect of the present invention, a substantiallyhaze-free BST thin film is provided. The BST thin film has a texturedstructure with a substantially uniform crystal orientation.

[0011] In another aspect of the present invention, the method of formingthe substantially haze-free BST film first comprises forming anucleation layer over a substrate assembly. Then, the BST film is formedover the nucleation layer, the BST film being formed having asubstantially uniform crystal orientation. In one preferred embodiment,the nucleation layer is NiO, and an orientation layer such as platinumis formed over the nucleation layer before forming the BST film. Theorientation layer preferably has a desired crystal orientation to inducethe same orientation in the subsequently formed BST film. In anotherpreferred embodiment, the nucleation layer is made of Ti, Nb or Mn tocompensate for defects in the subsequently formed BST film.

[0012] In another aspect of the present invention, a thin film structureis provided comprising a nucleation layer and a BST film over thenucleation layer having a substantially uniform crystal orientation. Inone embodiment, an orientation layer is preferably provided over thenucleation layer underneath the BST film. In another embodiment, the BSTfilm is directly on top of the nucleation layer.

[0013] In another aspect of the present invention, a method of forming aBST capacitor structure is provided. A first electrode material isformed over a substrate assembly, followed by forming a BST film overthe first electrode material. The BST film being formed has asubstantially uniform crystal orientation. A second electrode materialis then formed over the BST film. The first electrode material ispreferably formed in a vacuum at a temperature between about 500 and550° C., while the BST film is preferably formed at a temperaturegreater than about 580° C. The BST film is preferably deposited in avacuum chamber, with the first electrode material and the BST filmformed without a vacuum break in between.

[0014] In another aspect of the present invention, a capacitor structureis provided comprising a base layer, a bottom electrode formed over thebase layer, a BST film formed over the bottom electrode, and a topelectrode formed over the BST film. The BST film has a substantiallyuniform orientation, and preferably comprises between about 50 and 53.5atomic percent titanium. Preferably, a nucleation layer of NiO isprovided between the base layer, which is preferably polysilicon, andthe bottom electrode, which is preferably platinum. A second oralternative nucleation layer may be provided between the bottomelectrode and the BST film, and more preferably comprises a materialsuch as Ti, Mn or Nb.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015]FIG. 1 is a schematic view of the crystal structure of BST.

[0016]FIG. 2 is a schematic cross-sectional view of a capacitorstructure for use in a DRAM memory cell incorporating a BST thin film.

[0017]FIG. 3 is a schematic view of a MOCVD processing apparatus used todeposit a BST thin film.

[0018]FIG. 4A is a schematic cross-sectional view of a (100)-orientedBST thin film deposited over a platinum bottom electrode.

[0019]FIG. 4B is a schematic cross-sectional view of a polycrystallineBST thin film deposited over a platinum bottom electrode.

[0020]FIG. 5 is a schematic cross-sectional view depicting thedeposition of a BST thin film over a platinum bottom electrode using anucleation layer of NiO.

[0021]FIG. 6 is a schematic cross-sectional view depicting thedeposition of a BST thin film over a platinum bottom electrode using anucleation layer of Ti, Nb or Mn.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Briefly stated, the objects of the present invention areaccomplished by providing methods and apparatus that favor the growth oftextured BST films with a substantially uniform desired crystalorientation. The preferred embodiments of the present invention describea BST thin film formed as a capacitor in a DRAM memory cell. However, itwill be appreciated that the teachings disclosed herein are applicableto any method or device where a haze-free BST film with high dielectricconstant is desired. As used herein, the term BST can refer not only to(Ba,Sr)TiO₃, but also to BaTiO₃, SrTiO₃ and any modifications to thesematerials through isovalent substitution or the use of donor andacceptor dopants. Furthermore, the methods and apparatus taught hereinare applicable to other materials similar to BST where it is desired toreduce haze and maintain a high dielectric constant.

[0023]FIG. 1 illustrates the BST crystal structure. As can be seen, BSThas a cubic crystal structure, more particularly, a perovskite crystalstructure with Ba²⁺ and Sr²⁺ at the corners, O²⁻ at the faces and Ti⁴⁺at the center. The (100) planes of the BST crystal structure are shadedin FIG. 1. As described below, the illustrated embodiments of thepresent invention provide textured growth of the deposited BST filmalong (100) planes. However, this invention is not to be limited toproducing a BST film with only a substantially uniform (100)orientation. It will therefore be appreciated that the methods andprocessing conditions described herein are also applicable to producinga BST film with a desired crystal orientation along other planes in the{100} family, as well as planes in the {110}, {111} and other familiesof planes found in the BST crystal structure.

[0024]FIG. 2 illustrates a schematic capacitor structure 10 for use in aDRAM memory cell. At the bottom of the cell 10 is a base layer 12,preferably made of a material such as polysilicon. A bottom electrode orstorage node 14 is formed, preferably by chemical vapor deposition, overthe base material 12. This bottom electrode is preferably platinum (Pt)or ruthenium (Ru). Other electrodes, such as Ir, IrO_(X), RuO_(X)Pt—Rh,Mo and Pd may also be used. A thin film 16 of BST is deposited over thebottom electrode 14. A top electrode or cell electrode 18 is preferablydeposited by chemical vapor deposition over the BST thin film 16, and ispreferably made of material similar to the bottom electrode, such as Ptor Ru. In the exemplary capacitor structure 10, the bottom electrode 14is preferably grown to a thickness of about 400 Å. The BST film 16 grownover the bottom electrode 14 preferably has a thickness of about 150 to300 Å. At the top of the structure 10, the top electrode 18 preferablyhas a thickness of about 300 Å.

[0025] Optionally, a barrier layer may be formed between the polysiliconbase layer 12 and the electrode 14 to prevent interdiffusion between thelayers and the formation of SiO₂ on top of the electrode surface.Appropriate barrier layers include TiN/Ti, TiAlN, TaSiN, WSiN, and maybe formed by chemical vapor deposition or physical vapor deposition, aswould be known to one skilled in the art.

[0026] The BST film 16 is formed over the bottom electrode 14 preferablyusing metal-organic chemical vapor deposition (MOCVD). A schematic MOCVDapparatus 20 is illustrated in FIG. 3, including a chamber 21 with asubstrate assembly 22 located therein. Suitable BST sources, including abarium source 24, a strontium source 26, and a titanium source 28, areprovided into the chamber, as would be known to one skilled in the art.For example, suitable barium and strontium sources include Ba(THD)₂ andSr(THD)₂, where THD denotes 2,2,6,6-tetramethyl-3,5-heptanedionate.Suitable Ti sources include bis(isopropoxy)bis(2,2,6,6-tetramethyl-3,5-heptanedionate) titanium(Ti(O-i-Pr)₂(THD)₂). These precursors may be adducted with tetraglyme.Solvents used for these precursors may be butyl acetate ortetrahydrafuran, as would be known to one skilled in the art. A carriergas 33 such as Ar is preferably used to carry vapor from the sourcesinto the reaction chamber 21. Oxidizers 30 and 32 of O₂ and N₂O gas,respectively, are preferably used.

[0027] Deposition of the BST film 16 is preferably conducted in achamber 21 at a pressure of about 100 mtorr to 10 torr. In one preferredembodiment, it has been found that chamber temperatures greater thanabout 580° C. are effective in reducing haze. For high temperature BSTprocesses, deposition preferably occurs at a chamber temperature ofbetween about 600 and 680° C., and a substrate assembly temperature ofabout 500 to 580° C. For low temperature BST processing, depositionpreferably occurs at a chamber temperature of between about 400 and 500°C., and a substrate temperature of about 350 and 450° C. The BST film 16is preferably deposited at a rate of about 10 to 100 Å/min, morepreferably less than about 80 Å/min.

[0028] The resulting BST film 16 has a titanium concentration of betweenabout 50 and 53.5 atomic percent. It has also been found that whentitanium concentrations of about 50-52 atomic percent yield haze,increasing the titanium concentration to about 52-53 atomic percent canreduce haze. The ratio of Ba to Sr in the resulting film is preferablybetween about 70/30 to 50/50.

[0029]FIG. 4A illustrates a BST film 16 formed over a platinum electrode14. The conditions described above preferentially form the BST film 16with a substantially uniform orientation. For example, substantially allof the BST growth may be with a (100) orientation. In other words, thedirection of growth of the BST film 16 as indicated by arrow 15 isnormal to (100) planes in the BST film. This (100) orientation leads toa more uniform grain structure and a smoother surface, thereby producinga textured structure with a less cloudy or hazy appearance. By contrast,FIG. 4B illustrates a BST film 16 which is not uniformly oriented. Thestructure shown in FIG. 4B is polycrystalline, with growth occurring inmore than one direction and thus deviating from the preferredorientation. For example, when (100) is the desired orientation, thestructure shown in FIG. 4B may be caused by deviating growth with (110)or (111) orientations. This causes a more disrupted grain structure andan uneven surface, thereby leading to haze. It will be appreciated thatalthough the preferred embodiments describe growth with a substantiallyuniform (100) orientation, uniform growth with other orientations arecontemplated as well.

[0030] It has been found that the substantially uniform orientationillustrated in FIG. 4A is induced by the high temperatures, preferablyabove about 580° C., used for BST deposition, which favors equilibrium.Moreover, the slow deposition rate used, preferably less than about 80Å/min as described above, favors the formation of a more stable film,while the high Ti concentration in the BST produced, preferably betweenabout 50 and 53.5 atomic percent, also favors a more haze-free film.

[0031] As a modification to the embodiments described above, the use ofone or more nucleation layers either above or below the bottom electrode14, or both, favors a desired direction of crystal growth. For example,when the BST thin film 16 is grown over a bottom electrode 14 such asPt, a desired {100} orientation of the BST film can be favored by usinga thin NiO nucleation layer or similar material. Normal growth of Ptover a base material 12 such as silicon is along the {111} planes of Pt.This leads to the problem that BST films grown over a Pt electrode 14may not be {100} oriented, but rather {111} oriented.

[0032] As shown in FIG. 5, this problem is avoided by depositing a thinlayer 34 of NiO over the base material 12 to induce {100}-oriented Pt.NiO may be grown by sputtering or metal-organic chemical vapordeposition. The NiO nucleation layer 34 preferentially induces a highlytextured {100} orientation in the Pt layer 14. The Pt layer in turn actsas an orientation layer that induces the same orientation in thesubsequently deposited BST film 16. This is because both Pt and BST havesubstantially the same lattice constant of about 3.9 to 4 Å. Thus, the{100} orientation of the Pt layer 14 is preferentially transferred tothe BST film 16. It is also contemplated that other nucleation ororientation layers may be used to induce growth of subsequent layerswith other desired crystal orientations.

[0033] In another modification to the embodiments described above, hazecan also be prevented by providing a thin nucleation layer over thebottom electrode 14. As shown in FIG. 6, a thin layer 36 of materialsuch as Ti, Nb or Mn is grown over the bottom electrode 14. The layer 36is preferably deposited using a physical vapor deposition technique suchas sputtering to a thickness of less than about 50 Å. The layer 36preferentially induces a more uniform, haze-free BST film 16 bycompensating for defects formed in the subsequently deposited BST film.More particularly, the nucleation layer 36 can act as either a donor oracceptor dopant to correct for defects in the BST film. The nucleationlayer 36 also impacts the nucleation kinetics of the BST film to enablea more uniform growth.

[0034] In another embodiment, the properties of the BST film formedaccording to the embodiments described above can be improved by forminga smoother bottom electrode layer 14, which thereby leads to an improvedhaze-free BST film. In particular, by reducing the number of hillocksand increasing the smoothness of the bottom electrode layer 14, thedeposited BST film will experience less stress and have a more uniformgrowth. Preferably, the smoothness of the bottom electrode 14 isaccomplished by depositing the electrode at a high temperature close tothat of the BST deposition. More particularly, it has been found thatfor high temperature BST processing, depositing the bottom electrode attemperatures as high as about 500 to 550° C. reduces the stress in thesubsequently deposited BST film.

[0035] The bottom electrode 14 is also preferably deposited using aclustered deposition technique. By this technique, the bottom electrode14 is deposited under vacuum conditions, and then the BST film 16 isdeposited thereover without a vacuum break. By eliminating the vacuumbreak and forming both the bottom electrode and the BST film in aclustered tool, it has been observed that the number of hillocks in thebottom electrode 14 is reduced, while also reducing the stress in thelayer 14.

[0036] It should be appreciated that the embodiments described above arepurely exemplary, and various modifications can be made as would beknown to one of skill in the art. Accordingly, the scope of the presentinvention should be defined only by the claims that follow.

What is claimed is:
 1. A method of forming a haze-free BST film over asubstrate assembly, comprising: supplying BST sources into a chamber;and inducing textured growth of the BST film over the substrate assemblyin a substantially uniform desired crystal orientation.
 2. The method ofclaim 1, wherein inducing textured growth of the BST film includesdepositing the film at a rate of less than about 80 Å/min.
 3. The methodof claim 1, further comprising heating the chamber to a temperatureabove about 580° C. while inducing growth of the BST film.
 4. The methodof claim 1, wherein the BST film is grown using metal-organic chemicalvapor deposition (MOCVD).
 5. The method of claim 1, wherein theresulting BST film has a titanium concentration of about 50 to 53.5atomic percent.
 6. The method of claim 5, wherein the resulting BST filmhas a titanium concentration of about 52 to 53 atomic percent.
 7. Themethod of claim 1, wherein the BST film is grown in a substantiallyuniform {100} orientation.
 8. The method of claim 1, wherein inducingtextured growth of the BST film in a substantially uniform orientationcomprises: forming a layer having the desired crystal orientation overthe substrate assembly; and depositing the BST film in the desiredcrystal orientation over the layer.
 9. The method of claim 8, whereinthe layer having the desired orientation is made of a material selectedfrom the group consisting of Pt, Ru, RuO_(X), Ir, IrO_(X), Pt-Rh, Pd andMo.
 10. The method of claim 8, wherein inducing textured growth of theBST film in a substantially uniform orientation further comprisesforming a nucleation layer over the substrate assembly, wherein thelayer having the desired orientation is formed over the nucleationlayer.
 11. The method of claim 10, wherein the nucleation layercomprises NiO.
 12. The method of claim 11, wherein forming thenucleation layer of NiO induces a substantially uniform {100}orientation in the layer having the desired orientation.
 13. The methodof claim 1, wherein inducing textured growth of the BST film in asubstantially uniform desired crystal orientation comprises: forming anucleation layer over the substrate assembly; and depositing the BSTfilm over the nucleation layer.
 14. The method of claim 13, wherein thenucleation layer comprises a material selected from the group consistingof Ti, Mn and Nb.
 15. A method for forming a substantially haze-free BSTfilm, comprising: supplying BST sources into a chamber; heating thechamber to a temperature above about 580° C.; and depositing the BSTfilm at a rate of less than about 80 Å/min.
 16. A substantiallyhaze-free BST thin film having a textured structure with a substantiallyuniform crystal orientation.
 17. The BST thin film of claim 16, having asubstantially uniform {100} orientation.
 18. A method of forming asubstantially haze-free BST film over a substrate assembly, comprising:forming a nucleation layer over the substrate assembly; and forming aBST film over the nucleation layer, the BST film being formed having asubstantially uniform crystal orientation.
 19. The method of claim 18,further comprising forming an orientation layer over the nucleationlayer before forming the BST film.
 20. The method of claim 19, whereinthe orientation layer has a desired orientation to induce the sameorientation in the subsequently formed BST film.
 21. The method of claim19. wherein the orientation layer is selected from a group of materialsconsisting of Pt, Ru, RuO_(X), Ir, IrO_(X), Pt—Rh, Pd and Mo.
 22. Themethod of claim 19, wherein the nucleation layer comprises NiO.
 23. Themethod of claim 22, wherein the NiO layer induces a {100} orientation inthe subsequently formed orientation layer.
 24. The method of claim 18,wherein the nucleation layer compensates for defects in the BST film.25. The method of claim 24, wherein the nucleation layer comprises amaterial selected from the group consisting of Ti, Nb and Mn.
 26. Themethod of claim 18, wherein the BST film is formed by metalorganicchemical vapor deposition (MOCVD).
 27. The method of claim 18, whereinthe BST film is formed at a temperature greater than about 580° C. 28.The method of claim 18, wherein the BST film is deposited at a rate ofless than about 80 Å/min.
 29. A thin film structure, comprising: anucleation layer; and a BST film over the nucleation layer having asubstantially uniform crystal orientation.
 30. The thin film structureof claim 29, wherein the nucleation layer comprises NiO.
 31. The thinfilm structure of claim 29, further comprising an orientation layer overthe nucleation layer underneath the BST film to induce a desiredorientation in the BST film.
 32. The thin film structure of claim 31,wherein the orientation layer has a {100} orientation to induce a {100}orientation in the BST film.
 33. The thin film structure of claim 32,wherein the orientation layer is platinum.
 34. The thin film structureof claim 29, wherein the nucleation layer comprises a material selectedfrom the group consisting of Ti, Nb and Mn.
 35. The thin film structureof claim 29, wherein the nucleation layer has a thickness of less thanabout 50 Å.
 36. The thin film structure of claim 29, wherein the BSTfilm has a thickness of about 150 to 300 ÅA.
 37. A method of forming aBST capacitor structure, comprising: forming a first electrode materialover a substrate assembly; forming a BST film over the first electrodematerial, the BST film being formed having a substantially uniformcrystal orientation; and forming a second electrode material over theBST film.
 38. The method of claim 37, further comprising eliminatinghillocks from the first electrode material.
 39. The method of claim 37,wherein the first electrode material is formed in a vacuum at atemperature between about 500 and 550° C. to reduce stress in thesubsequently formed BST film.
 40. The method of claim 37, wherein theBST film is formed at a temperature greater than about 580° C.
 41. Themethod of claim 37, wherein the BST film is deposited in a vacuumchamber, and the first electrode material and the BST film are formedwithout a vacuum break in between.
 42. A capacitor structure,comprising: a base layer; a bottom electrode formed over the base layer;a BST film formed over the bottom electrode, the BST film having asubstantially uniform crystal orientation; and a top electrode formedover the BST film.
 43. The capacitor structure of claim 42, wherein theBST film comprises between about 50 and 53.5 atomic percent titanium.44. The capacitor structure of claim 42, wherein the BST film comprisesbetween about 52 and 53 atomic percent titanium.
 45. The capacitorstructure of claim 42, further comprising a nucleation layer between thebase layer and the bottom electrode.
 46. The capacitor structure ofclaim 45, wherein the nucleation layer is made of NiO.
 47. The capacitorstructure of claim 46, wherein the bottom electrode is made of platinum.48. The capacitor structure of claim 42, further comprising a nucleationlayer between the bottom electrode and the BST film.
 49. The capacitorstructure of claim 48, wherein the nucleation layer is made of amaterial selected from the group consisting of Ti, Nb and Mn.
 50. Thecapacitor structure of claim 42, wherein the base layer comprisespolysilicon.
 51. The capacitor structure of claim 42, wherein the bottomelectrode is selected from the group of materials consisting of Pt, Ru,Ir, IrO_(X), RuO_(X)Pt—Rh, Mo and Pd.
 52. The capacitor structure ofclaim 42, wherein the top electrode is selected from the group ofmaterials consisting of Pt, Ru, Ir, IrO_(X), RuO_(X)Pt—Rh, Mo and Pd.53. A method of reducing haze in a BST film, comprising increasing thetitanium concentration of the film to about 52 to 53 atomic percent.